Gamut is a way to encode a color, as well as the sum of colors that a technical system can produce. In the display technology, due to the range of colors (i.e. gamut) that various color devices can display, Gamut Mapping techniques and algorithms are required for color image reproduction. Gamut mapping generally refers to mapping a source gamut of an input image signal to a target gamut suitable for a display device to display the image.
Color space, also known as color models, is used to describe color in some generally accepted ways. In order to describe colors more appropriately, the International Commission on Illumination (CIE) has proposed various color spaces such as RGB color space, XYZ color space, Lab color space, LCH color space and the like.
RGB (red, green and blue) color space is a color space defined according to the human eye, through the three basic colors of different degrees of superposition to produce a variety of different colors, usually, the numerical values of the three basic colors are all [0,255]. In the case of the existing display device, the input RGB digital value needs to be gamma-corrected, and the RGB digital value needs to be converted into the RGB optical value.
The XYZ color space is a color space composed of three elements of tristimulus values X, Y and Z. The tristimulus values X, Y and Z can be obtained by converting the RGB optical values, as we all know, the conversion relationship can generally be expressed as
            [                                    X                                                Y                                                Z                              ]        =          M      ×              [                                            R                                                          G                                                          B                                      ]              ,wherein R, G, B are the RGB optical value, M is the particular transformation matrix. Correspondingly, an inverse transform relationship may be obtained by the inverse matrix of M.
Lab color space is a device-independent color space, which is a color space composed of three elements of L, a, and b; the L component in the Lab color space is used to represent the brightness of the pixel, in the range of [0,100], from pure black to pure white; a represents the range from red to green, the range of values is [127, −128]; b represents the range from yellow to blue, and the value range is [127, −128].
The XYZ tristimulus values can be converted into the L*a*b* value of the Lab color space by the formula:
            L      *        =                  116        ⁢                                  ⁢                  f          ⁡                      (                          Y                              Y                n                                      )                              -      16                  a      *        =          500      ⁡              [                              f            ⁡                          (                              X                                  X                  n                                            )                                -                      f            ⁡                          (                              Y                                  Y                  n                                            )                                      ]                        b      *        =          200      ⁡              [                              f            ⁡                          (                              Y                                  Y                  n                                            )                                -                      f            ⁡                          (                              Z                                  Z                  n                                            )                                      ]                        f      ⁡              (        t        )              =          {                                                  t                              1                /                3                                                                        (                              t                >                                                      (                                          6                      29                                        )                                    3                                            )                                                                                                            1                  3                                ⁢                                                      (                                          29                      6                                        )                                    2                                ⁢                t                            +                              4                29                                                                        (              others              )                                          
when the maximum brightness normalized to 100, Xn, Yn and Zn are general defaulted as 95.047,100,108.883.
Accordingly, the L*a*b value of the Lab color space can be converted into the XYZ tristimulus value by the formula:
            f      ⁡              (                  Y                      Y            n                          )              =                  L        +        16            116                  f      ⁡              (                  X                      X            n                          )              =                  a        500            +              f        ⁡                  (                      Y                          Y              n                                )                                f      ⁡              (                  Z                      Z            n                          )              =                  f        ⁡                  (                      Y                          Y              n                                )                    -              b        /        200                  t    =          {                                                                  f                ⁡                                  (                  t                  )                                            ^              3                                                          (                                                f                  ⁡                                      (                    t                    )                                                  >                                  6                  29                                            )                                                                                          (                                                      f                    ⁡                                          (                      t                      )                                                        -                                      4                    29                                                  )                            /                              (                                                      (                                          1                      3                                        )                                    *                                                            (                                              29                        /                        6                                            )                                        ^                    2                                                  )                                                                        (              others              )                                          
The LCH color space is a color space composed of three elements of L, C and H. L represents the brightness and is the same as L* in the Lab color space. C is the color saturation and H is the hue angle.
As shown in FIG. 1, it is a constant hue flat view of the Lab color space. Any color point P and brightness axis in the Lab color space can be used as a plane to convert the three-dimensional pace into a two-dimensional plane. The color point P hue angle
      H    =          arctan      ⁢                          ⁢                        b          *                          a          *                      ,the plane coordinates (C, L) can be used to measure the color point position, at this time:
      H    =          arctan      ⁢                          ⁢                        b          *                          a          *                          C    =                  a        ⁢                  *          2                ⁢                  +          b                ⁢                  *          2                          L    =          L      *      
In the FIG. 1, The vertical axis of the two-dimensional plane CL represents the brightness L, the horizontal axis represents the color saturation C, the position of the color point P is measured by coordinates (C, L), the color point P is located in the gamut defined by the color point D and the origin O and L axis.
For the gamut mapping algorithm, in the process of mapping the large gamut to the small gamut, it can be analyzed in conjunction with the constant hue plane (LC section) of the Lab color space. The common algorithm for the color points outside the small gamut are mapped to a small gamut boundary, there will always be a region or a line segment mapped to the same point phenomenon, that is, overlay mapping problem. Directly leads to the level of image loss detail after mapping, resulting in the phenomenon of halo noise.
For the ultra-gamut problem, the traditional algorithm outputs the color in the gamut directly as it is, cutting all the colors outside the gamut to the target color gamut boundary. There are two common methods as shown in FIG. 2 and FIG. 3.
As shown in FIG. 2, FIG. 2 is an algorithm diagram of the HPMINDE, the vertical axis represents the brightness L, the horizontal axis represents the color saturation C, the triangular area enclosed by the color point D and the origin O and the L axis represents the source gamut, the triangular area enclosed by the color point T and the origin O and L axis represents the target gamut. The mapping principle of the method is to ensure that the color difference value of the color before and after the mapping is the minimum: the color of the ultra-gamut is mapped to the target gamut boundary according to the principle of the minimum color difference, that is, the color point P′ at the shortest Euclidean distance.
As shown in FIG. 3, FIG. 3 is an algorithm diagram of a gamut mapped along a fixed point Lfocal direction, and the triangular area enclosed by the color point D and the origin O and the L axis represents the source gamut, the triangular area inside the source gamut represents the target gamut. The mapping principle of this method is to map the color of the ultra-gamut along the direction of the fixed point Lfocal on the brightness axis and map the color point P of the ultra-gamut to the target gamut boundary sequentially to obtain the color point P′.
The features of the existing method are as follows: the color point in the gamut is output as it is to ensure accurate color rendering in the gamut; the color point of out-of-gamut is mapped on the gamut boundary according to different principles. The disadvantages of the existing methods are as follows: in Method 1, the color in the K region will be mapped to the cusp T, and the colors on the midline PP′ in Method 2 are all mapped to the point P′. Mapping the color of the image in this area loses the level of detail, creating blurring and Halo noise.